A pattern of inhibition by Group D streptococci on S. mutans has been determined by the use of simple matching, single linkage computer analyses. S. mutagen Bratthall types a and d form one cluster. Two other clusters were generated. Independently, the challenger strains formed three clusters. The Bratthall types fall into three groups related to carbohydrate antigens. The physicochemical nature of inhibition will be investigated and the results will be analyzed with respect to antigen types of the sensitive S. mutans. Four group D streptococcal challengers which inhibit a wide range of S. mutans and whose antagonism is resistant to proteolytic enzymes will serve as donors in transformation experiments with S. sanguis strain challis. Transformants which exhibit antagonism on S. mutans characteristic of the donors will be employed in competition tests employing germ-free rats.